Method for compensating for the movement of the antenna for a sonar

ABSTRACT

The invention relates to towed sonars. It consists in compensating electronically for the movement of the antenna in order to suppress the Doppler shift of the fixed echoes. In order to do that in a towed linear antenna (203) comprising N sensors (205), M (M&lt;N) successive sensors are selected which are switched electronically in order to make them travel synthetically over the antenna in the direction opposite to the movement of the latter at a speed equal to twice the forward speed of the antenna. In order not to have too large a number of physical sensors, the signals from the latter are interpolated in order to obtain synthetic sensors which are sufficient in number to reduce the increment of the movement of the synthetic sub-antenna in such a way as to suppress the parasitic lobes due to the incremental nature of this movement. It makes it possible to increase the sensitivity of sonars without increasing their power. FIG. 2.

The present invention relates to methods which make it possible tocompensate, in sonars, for the movements of the antenna which riskdisturbing the reception of the acoustic signals.

It is known, in order to detect a moving target with a sonar, to use theDoppler effect by sending out an acoustic pulse whose frequency band issmaller than the Doppler shift likely to affect the echoes originatingfrom moving targets. On reception, the received signals are correlatedwith several copies of the signals sent out, each copy corresponding toa different Doppler shift. The type of correlation obtained with thecopy which corresponds to a Doppler shift identical or substantiallyidentical to that of the target makes it possible to locate this targetin distance, as well as to determine its radial speed with respect tothe sonar. This method is entirely compatible with the other signalprocessing systems, in particular those which consist in formingdirectional receiving channels. This method makes it possible to improvethe contrast between the useful echo which is affected by a Dopplershift and the other echoes originating from the reverberation as muchfrom the bottom and from the surface of the sea, as from the volumeitself of the underwater medium whose non-uniformities give rise to adiffuse ambient echo which is very troublesome for a simple sonar. Infact, the echoes due to reverberation are not affected by a Dopplershift since the elements at the origin of this reverberation do notmove, which makes it possible to eliminate them. This assumes that thetarget is moving, which is generally the case.

Very often the sonar is carried, or towed, by a boat which is moving ata speed which is not negligible with respect to that of the target. Inthis case the echoes due to reverberation are affected by a Dopplershift originating from the speed of the sonar itself. As transmission ingeneral covers a wide angular sector, there are always reverberationsources whose radial speed with respect to the sonar is substantiallyequal to that of the moving target. In these conditions the echo/noisecontrast is appreciably limited by these reverberation sources, even ifthese sources are outside the main lobe of the receiving channel usedsince, as is known, there are always relatively significant secondarylobes in the diagram of such a channel.

In order not to be too much disturbed by such an effect, it would benecessary for the target to move at a speed at least higher than twicethe speed of the vessel which is carrying the sonar. This would lead tovery seriously limiting the speed of the vessel and thus its operationalcapabilities. If, moreover, the target is a torpedo which is headingtowards the boat, it is easy to imagine the catastrophic consequences ofsuch a speed reduction on the capability of escaping this very torpedo.

In order to overcome the problems related to movements of the sonar, theinvention proposes a method of compensating for the movement of theantenna for a sonar, in which the antenna comprises a set of transducersand is driven in a translational movement, characterized principally inthat the signals from these transducers are switched in order to obtaina synthetic movement of the antenna making it possible to compensate forthe Doppler shift applied to the fixed echoes by the said translationalmovement.

Other features and advantages of the invention will appear clearly inthe following description, given by way of non-limiting example withregard to the attached figures which represent:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, a diagram of reception from a target and from a reverberationcell;

FIG. 2, a boat towing a sonar antenna;

FIG. 3, the constructional diagram of the virtual antennae of a sonaraccording to the invention;

FIG. 4, the variation of phase over time of the reception signal;

FIG. 5, the spectrum of this signal; and

FIG. 6, the block diagram of a device for implementing the invention.

In FIG. 1, a sonar 101, carried by a boat moving with a speed V_(A)receives the echo from a target 102 situated at an azimuth Θcorresponding to the main lobe of a receiving channel 104.

This target is moving with a speed V_(c) forming an angle Θ_(c) withrespect to the axis of the receiving channel 104. The sonar furtherreceives echoes from various places in the sea forming reverberationcells. One of the latter, 103 for example, lies at an azimuth Θ_(R) withrespect to V_(A), this azimuth corresponding to a secondary lobe 105 ofthe main receiving channel 104.

Starting from the known formula giving the Doppler shift which is equal:##EQU1## in which V_(r) is the radial speed between the target and thesonar, c the speed of sound in water, and f₀ the transmission frequencyof the sonar, the Doppler shift will be the same for the target and thereverberation cell 103 when the parameters of these two echo sourcessatisfy the following equation:

     V.sub.A cos Θ.sub.r =V.sub.c cos Θ.sub.c +V.sub.A cos Θ(2)

As the reverberation cells are situated throughout the space surroundingthe sonar, particularly those originating from volume reverberation,this equality has every chance of being satisfied in one or moredirections corresponding to secondary receiving lobes of the sonar. Evenif these lobes are relatively attenuated with respect to the main lobe,the result is finally heavily disturbed.

In FIG. 2 is represented a towing vessel 201 which includes, under itshull., a sonar transmitter 202 which radiates, in a substantiallyomnidirectional way, throughout the undersea space. The vessel istowing, with the aid of a towing cable 204, a linear sonar antenna 203formed by a set of sensors 205 regularly spaced by a distance d alongthe antenna.

As explained above, the vessel is advancing in a direction substantiallyparallel to its axis with a speed V_(A). It carries along thetransmitter 202 and the receiving antenna 203, needless to say. In theseconditions the echoes received by the antenna 203 originating frompulses of duration T sent out by the transmitter 202, exhibit a Dopplershift even when they originate from reverberation on fixed obstacles,whose value corresponds to a relative speed 2 V_(A). The coefficient 2stems from the fact that the transmitter and the receiver are bothadvancing at the speed V_(A).

In order to eliminate this Doppler shift, and the drawbacks which itexhibits, it would be necessary for the receiving antenna to move in thedirection opposite to the movement of the boat with a speed V_(A). Thisis manifestly not possible since, even if the towing cable is left topay out freely, the antenna would remain at best immobile in the sea andthere would thus be a Doppler shift corresponding to a speed V_(A) dueto the movement of the transmitter. In order to obtain completecompensation, the invention proposes to select at least one set of Mconsecutive sensors among the N sensors constituting the antenna and tomove this set electronically along the antenna 203 towards the rear ofthe latter at a speed equal to 2 V_(A) with respect to the antenna.Hence this set of N sensors is moving with respect to the marine mediumat a speed V_(A) directed in the opposite sense to the forward movementof the boat, which makes it possible to compensate completely for theDoppler shift introduced on the fixed echoes by the movement of the boatand of the receiving antenna. Everything happens as if the sensors ofthe receiving antenna were placed on a belt moving at a speed -2 V_(A)with respect to the receiving antenna. When the synthetic movement ofthis set of M sensors has brought the sliding sub-antenna to the rearextremity of the physical antenna, it departs again to the frontextremity of the latter, which is done instantaneously since simpleelectronic switching is involved.

The processing of the signal consists, as was seen above, in addition tothe formation of channels, in correlating the signals received with acopy of the signal sent out. This signal being a pulse of duration T,correlation makes it necessary to use a signal received during this sameduration T. As the M sensors are moving synthetically along the antennait is therefore necessary for the sub-antenna thus selected to besufficiently small in order not to overshoot the rear extremity of theantenna before the end of the duration T. This imposes a maximum valueon M as a function of T, which is given by the relationship:

     Nd≧Md+2 V.sub.A T                                  (3)

Needless to say it will be beneficial to arrange for this inequality tobe an equality so as to have a maximum number of sensors used in orderto obtain the best received signal. This case is represented in thediagram of FIG. 2 in which a first subassembly is seen to depart fromthe start of the physical antenna to arrive at the end of the latter atthe expiry of a time T. This movement is represented by the shadedsurface 301.

When this first subantenna has arrived at the end, it therefore departsagain to the start along the dotted line 311.

This processing will, needless to say, give output signals every Tseconds, which correspond to echoes distributed in time every 2 V_(A) Tmeters, which leaves observation holes in the undersea space. Theproblem is identical when this sliding antenna system according to theinvention is not used. In the usual practice, successive identicalprocessings, staggered over the duration T, are carried out on the setof sensors of the antenna. According to the invention the same practicewill be used, but by selecting several sliding subantennae which willdepart from the start of the antenna at regularly staggered instantsduring this duration T. The number of subantennae thus selected willessentially be a function of the processing capacity of the computersystem which is carrying out all these calculations, with regard to thecoverage which it is desired to obtain, having regard to the fact thatan echo is not a point source. As there is no necessity to have astrictly continuous coverage, in practice an overlap of 75% is oftensufficient and then corresponds to four subantennae 301 to 304 whichdepart one after the other from the start of the physical antenna duringthe duration T. At the expiry of the duration T, the antenna 301 againdeparts from the start of the antenna as represented by referencenumeral 321 in FIG. 3. Put another way, that means that, generally, theinstant of arrival of a pulse on the antenna does not coincide with theinstant at which the subantenna is found at the start of the antenna.Hence it is necessary to process several subantennae during the durationT.

It has been assumed implicitly up to now that in order to obtain thiscompensation for the movement it was possible at any instant to selectthe correct sensors at the correct places to constitute a subantennamoving with the correct speed. As the processing is carried outdigitally, as is now usual, on signal samples picked up on the sensors,a discrete number of sensors distributed along the antenna will besatisfactory. Nevertheless, as the sampling is sufficiently rapid totake account, on the one hand, of the Nyquist criterion, and, on theother hand, of certain limitations which will be seen later in the text,and as, moreover, it is necessary to anticipate that the speed of theboat may be variable, the subantenna is made to undergo a-syntheticmovement formed by a series of rearward jumps by a distance 1 every dtseconds. This movement is partially represented by the crenellated part312 in FIG. 3. Having regard to the orders of magnitudes currently used,it is difficult, as is seen on the figure, to have a sufficient numberof sensors to select those situated at the correct places.

This leads to an interpolation therefore being effected against severalsuccessive sensors in order to obtain the signal corresponding to anintermediate sensor situated at the appropriate place. This is a currenttechnique in the state of the art and presents no difficulty.

In these conditions, and as the real sensors are undergoing a continuousphysical movement, the signals originating from a target situated at anazimuth Θ and under way at a speed the projection of which on thedirection e is equal to V_(c), will have a phase, if f₀ is the centralfrequency of the pulse sent out and c the speed of sound in the marinemedium, which will depend on time according to the formula: ##EQU2##

The corrector term φ(t), the variation in which is represented in FIG.4, stems precisely from the incremental character of the compensationfor the movement of the antenna.

The spectrum of the received signal, which is represented qualitativelyin FIG. 5, therefore comprises image lobes whose amplitude is given bythe formula:

    (5) A=sin (π 1.cos Θ.of/c)/[π.(1+1. cos Θ.of/c)]

and whose position is given by the formula: ##EQU3## These lobes aretherefore spaced every 1/dt, as will easily be noted in FIG. 5. FIG. 5,needless to say, represents only the main lobe and the two secondarylobes which are of the most significant order.

It is therefore appropriate, in order not to spoil the beneficialresults of the invention by disturbances contributed by these imagelobes, to eliminate their influence. When the influence of the lobes oforder +1 and -1 has been eliminated, the influence of the lobes ofhigher order will automatically have been eliminated.

This elimination is done by the choice of 1 according to one of the twofollowing criteria, taking the least stringent:

The amplitude of the lobes can be reduced so that they are lower thanthe desired spectral level Ns for these lobes, which dictates that 1satisfies the relationship:

     20 log (sin π(1. cos Θ.of/c)/π(1+1. cos Θof/c)≧Ns (7)

For a currently sought value for Ns of -40 dB, 1 must then be less than1/100th of the transmission wavelength in the case in which cos Θ=1,which is a small and stringent value.

The other way of eliminating the influence of the image lobe is tochoose 1 so that the position of this image lobe falls outside thereceiving band Br of the sonar, which, since 1/dt must be greater thanor equal to Br, leads to the following relationship being satisfied:

     1≦2V.sub.A /Br                                     (8)

For example, taking a sonar operating at f₀ =3000 Hz for detectingtargets moving at a maximum speed of 30 m/s, corresponding to a band Brequal to 240 Hz for a speed of sound in water of 1500 m/s, and for aspeed of the boat V_(A) =10 m/s, a maximum value for 1 is obtained equalto 1/6th of the wavelength. It is noted that this value is much less ofa constraint than the preceding one. In the majority of cases this willtherefore be the criterion which will be chosen.

As it is current practice in the state of the art to place the sensorswith a spacing equal to 1/2 wavelength, it can thus be seen that moreoften than not the interpolation described above will have to be carriedout. This interpolation is done in a known way from nM_(i) sensors whichare closest to the virtual sensor whose position has to be interpolated.This number M_(i) depends (in a known way) essentially on the pitch d ofthe physical sensors and of the angular sector for reception, so as tohave a sufficient number of samples to interpolate validly. Therelationship (3) for the number of physical sensors to be used thenbecomes the relationship:

     Nd≧Md+2V.sub.A T+M.sub.i d                         (9)

Having regard to the orders of magnitudes used, nothing much changes,since M_(i) is markedly lower than M in practice.

The processing is then carried out according to the block diagramrepresented in FIG. 6. For P subantennae there are P processing channelseach including three steps:

in a first step, the subantenna is made up by choosing the physical orvirtual sensors which are necessary to obtain compensation for theDoppler effect:

in a second step, the receiving channels are formed;

in a third step, the correlation of the signals from these channels withthe copies of the signals sent out by the transmitter of the sonar iscarried out.

The steps of formation of channels and of correlation are entirelyconventional. The one corresponding to making up a subantenna of rank kfor each synthetic displacement increment of duration dt breaks downinto three sub-steps:

in a first step, the position of the M virtual (possibly real) sensorsto be used is calculated, taking the relationship: ##EQU4## n is thecurrent index which determines the position of the subantenna at everyinstant on the physical antenna;

in a second sub-step, the M_(i) physical sensors are selected whichcorrespond to each virtual sensor whose position has been determined inthe first substep, as well as the M_(i) interpolation coefficients to beapplied to the signals from these physical sensors in order to obtainthe signal from the virtual sensor;

in a third sub-step the M signals from the subantenna k arereconstituted by interpolation.

All this processing is carried out digitally as is the practicenowadays, with sampling of the physical sensors at a frequency fe whichis sufficient to be able to form the channels and such that, moreover,fe is higher than 1/dt.

Again taking the figures of the embodiment example given above, for alinear antenna including 200 sensors spaced by 0.2 m and moving at aspeed of 10 m/s (20 knots) and with a pulse having a duration of 1second, each sliding subantenna comprises 88 virtual sensors, each ofthese sensors being reconstituted from 12 physical sensors. Needless tosay, the same physical sensor can be used to reconstitute severalvirtual sensors.

By way of a variant, and for relatively low interpolation values, itwould be possible to use a sufficient number of physical sensorsdirectly. This is particularly advantageous for low transmissionfrequencies. Thus, in the example described above, if the transmissionfrequency is taken to be 1000 Hz it is possible to directly use thesignals from the physical sensors without interpolation, at the givenspeed of the boat.

As is well known in the art it is possible, needless to say, topermutate the two steps of formation of channels and of correlation.

The method according to the invention thus makes it possible tocompensate for the Doppler effect due to reverberation, without beingobliged to resort to an increase in the spatial rejection of thereceiving channels, which is always difficult and costly, or evenimpossible, to obtain. Moreover, since the Doppler shift has beencancelled out, the range of shift for the targets is itself alsoreduced, which makes it possible to reduce the number of copies used forthe correlation.

Finally, it is quite clear that, although the invention has beendescribed in the case of a sonar antenna with relatively point-liketransmission and of a towed linear receiving antenna, on which thecompensation for the movement is carried out, the invention extends toall the types of sonars in which it is possible to compensate for themovement of the antenna or antennae in this way. In particular,so-called active towed linear antennae are sometimes used, in which thesonar transmitter is situated within these antennae. It is then possibleto carry out the compensation within the transmission and even possiblyto distribute this compensation between the transmission and thereception.

We claim:
 1. A method of compensation for the movement of an antenna forsonar, wherein the antenna includes a set of transducers drawn along ina translational movement, comprising the steps of:outputting signalsfrom said transducers; switching said output signals from saidtransducer in order to obtain a synthetic movement of the antennawhereby a Doppler shift applied to fixed echoes by said translationmovement is compensated.
 2. The method according to claim 1 wherein theantenna is a towed linear antenna having a set of transducers regularlydistributed along said antenna and wherein at least one subset ofsuccessive sensors is selected, which said at least one subset is madeto slide synthetically in an incremental manner along the antenna in thedirection opposite to movement of said antenna.
 3. The method accordingto claim 2, wherein the signals from selected physical sensors areinterpolated in order to obtain synthetic sensors interspaced betweentwo successive physical sensors so as to reduce the increment of thesynthetic movement in order to reduce the effect of image lobes of anoutput signal originating from the incremental nature of the movement.4. A method according to one of claims 2 or 3, wherein several subsetsof successive sensors are selected, which said subsets of successivesensors are made to depart successively from the start of the antenna ina staggered, in time, manner in order to cover an entire field ofobservation of the sonar.
 5. A method according to any one of claims 2or 3 wherein the linear antenna is a receiving antenna for receiving anecho of defined duration and a number of selected transducers isselected so that said selected transducers travel over the whole of theantenna during said duration.